/*

 * jdatasrc.c

 *

 * Copyright (C) 1994, Thomas G. Lane.

 * This file is part of the Independent JPEG Group's software.

 * For conditions of distribution and use, see the accompanying README file.

 *

 * This file contains decompression data source routines for the case of

 * reading JPEG data from a file (or any stdio stream).  While these routines

 * are sufficient for most applications, some will want to use a different

 * source manager.

 * IMPORTANT: we assume that fread() will correctly transcribe an array of

 * JOCTETs from 8-bit-wide elements on external storage.  If char is wider

 * than 8 bits on your machine, you may need to do some tweaking.

 */



/* this is not a core library module, so it doesn't define JPEG_INTERNALS */

#include "jinclude.h"

#include "jpeglib.h"

#include "jerror.h"





/* Expanded data source object for stdio input */



typedef struct {

  struct jpeg_source_mgr pub;	/* public fields */



  FILE * infile;		/* source stream */

  JOCTET * buffer;		/* start of buffer */

  boolean start_of_file;	/* have we gotten any data yet? */

} my_source_mgr;



typedef my_source_mgr * my_src_ptr;



#define INPUT_BUF_SIZE  4096	/* choose an efficiently fread'able size */





/*

 * Initialize source --- called by jpeg_read_header

 * before any data is actually read.

 */



METHODDEF void

init_source (j_decompress_ptr cinfo)

{

  my_src_ptr src = (my_src_ptr) cinfo->src;



  /* We reset the empty-input-file flag for each image,

   * but we don't clear the input buffer.

   * This is correct behavior for reading a series of images from one source.

   */

  src->start_of_file = TRUE;

}





/*

 * Fill the input buffer --- called whenever buffer is emptied.

 *

 * In typical applications, this should read fresh data into the buffer

 * (ignoring the current state of next_input_byte & bytes_in_buffer),

 * reset the pointer & count to the start of the buffer, and return TRUE

 * indicating that the buffer has been reloaded.  It is not necessary to

 * fill the buffer entirely, only to obtain at least one more byte.

 *

 * There is no such thing as an EOF return.  If the end of the file has been

 * reached, the routine has a choice of ERREXIT() or inserting fake data into

 * the buffer.  In most cases, generating a warning message and inserting a

 * fake EOI marker is the best course of action --- this will allow the

 * decompressor to output however much of the image is there.  However,

 * the resulting error message is misleading if the real problem is an empty

 * input file, so we handle that case specially.

 *

 * In applications that need to be able to suspend compression due to input

 * not being available yet, a FALSE return indicates that no more data can be

 * obtained right now, but more may be forthcoming later.  In this situation,

 * the decompressor will return to its caller (with an indication of the

 * number of scanlines it has read, if any).  The application should resume

 * decompression after it has loaded more data into the input buffer.  Note

 * that there are substantial restrictions on the use of suspension --- see

 * the documentation.

 *

 * When suspending, the decompressor will back up to a convenient restart point

 * (typically the start of the current MCU). next_input_byte & bytes_in_buffer

 * indicate where the restart point will be if the current call returns FALSE.

 * Data beyond this point must be rescanned after resumption, so move it to

 * the front of the buffer rather than discarding it.

 */



METHODDEF boolean

fill_input_buffer (j_decompress_ptr cinfo)

{

  my_src_ptr src = (my_src_ptr) cinfo->src;

  size_t nbytes;



  nbytes = JFREAD(src->infile, src->buffer, INPUT_BUF_SIZE);



  if (nbytes <= 0) {

    if (src->start_of_file)	/* Treat empty input file as fatal error */

      ERREXIT(cinfo, JERR_INPUT_EMPTY);

    WARNMS(cinfo, JWRN_JPEG_EOF);

    /* Insert a fake EOI marker */

    src->buffer[0] = (JOCTET) 0xFF;

    src->buffer[1] = (JOCTET) JPEG_EOI;

    nbytes = 2;

  }



  src->pub.next_input_byte = src->buffer;

  src->pub.bytes_in_buffer = nbytes;

  src->start_of_file = FALSE;



  return TRUE;

}





/*

 * Skip data --- used to skip over a potentially large amount of

 * uninteresting data (such as an APPn marker).

 *

 * Writers of suspendable-input applications must note that skip_input_data

 * is not granted the right to give a suspension return.  If the skip extends

 * beyond the data currently in the buffer, the buffer can be marked empty so

 * that the next read will cause a fill_input_buffer call that can suspend.

 * Arranging for additional bytes to be discarded before reloading the input

 * buffer is the application writer's problem.

 */



METHODDEF void

skip_input_data (j_decompress_ptr cinfo, long num_bytes)

{

  my_src_ptr src = (my_src_ptr) cinfo->src;



  /* Just a dumb implementation for now.  Could use fseek() except

   * it doesn't work on pipes.  Not clear that being smart is worth

   * any trouble anyway --- large skips are infrequent.

   */

  if (num_bytes > 0) {

    while (num_bytes > (long) src->pub.bytes_in_buffer) {

      num_bytes -= (long) src->pub.bytes_in_buffer;

      (void) fill_input_buffer(cinfo);

      /* note we assume that fill_input_buffer will never return FALSE,

       * so suspension need not be handled.

       */

    }

    src->pub.next_input_byte += (size_t) num_bytes;

    src->pub.bytes_in_buffer -= (size_t) num_bytes;

  }

}





/*

 * An additional method that can be provided by data source modules is the

 * resync_to_restart method for error recovery in the presence of RST markers.

 * For the moment, this source module just uses the default resync method

 * provided by the JPEG library.  That method assumes that no backtracking

 * is possible.

 */





/*

 * Terminate source --- called by jpeg_finish_decompress

 * after all data has been read.  Often a no-op.

 *

 * NB: *not* called by jpeg_abort or jpeg_destroy; surrounding

 * application must deal with any cleanup that should happen even

 * for error exit.

 */



METHODDEF void

term_source (j_decompress_ptr cinfo)

{

  /* no work necessary here */

}





/*

 * Prepare for input from a stdio stream.

 * The caller must have already opened the stream, and is responsible

 * for closing it after finishing decompression.

 */



GLOBAL void

jpeg_stdio_src (j_decompress_ptr cinfo, FILE * infile)

{

  my_src_ptr src;



  /* The source object and input buffer are made permanent so that a series

   * of JPEG images can be read from the same file by calling jpeg_stdio_src

   * only before the first one.  (If we discarded the buffer at the end of

   * one image, we'd likely lose the start of the next one.)

   * This makes it unsafe to use this manager and a different source

   * manager serially with the same JPEG object.  Caveat programmer.

   */

  if (cinfo->src == NULL) {	/* first time for this JPEG object? */

    cinfo->src = (struct jpeg_source_mgr *)

      (*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_PERMANENT,

				  SIZEOF(my_source_mgr));

    src = (my_src_ptr) cinfo->src;

    src->buffer = (JOCTET *)

      (*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_PERMANENT,

				  INPUT_BUF_SIZE * SIZEOF(JOCTET));

  }



  src = (my_src_ptr) cinfo->src;

  src->pub.init_source = init_source;

  src->pub.fill_input_buffer = fill_input_buffer;

  src->pub.skip_input_data = skip_input_data;

  src->pub.resync_to_restart = jpeg_resync_to_restart; /* use default method */

  src->pub.term_source = term_source;

  src->infile = infile;

  src->pub.bytes_in_buffer = 0; /* forces fill_input_buffer on first read */

  src->pub.next_input_byte = NULL; /* until buffer loaded */

}

